Method and device for treating a carbon dioxide-containing gas stream

ABSTRACT

The invention relates to a method and an apparatus for treating a carbon dioxide-containing gas stream. Precompressed raw gas stream ( 1 ) is partially liquefied in a cryogenic carbon dioxide purification stage ( 2, 3, 4 ). Part of the resultant liquid is used to obtain a gas stream having an elevated carbon dioxide content ( 7 ). From the non-liquefied raw gas, a gas stream having a reduced carbon dioxide content is obtained. This vent gas stream is expanded and the refrigeration generated is recovered for cooling the raw gas stream. The carbon dioxide gas stream is compressed ( 8 ) to a final pressure and fed to further utilization and/or storage. Another part of the liquid from the cryogenic carbon dioxide purification stage is fed in a liquid phase ( 9 ) to further utilization and/or storage ( 10 ).

SUMMARY OF THE INVENTION

The invention relates to a method for treating a carbondioxide-containing gas stream (raw gas stream), in particular a carbondioxide-containing gas from a large-scale furnace plant. Theprecompressed raw gas stream is partially liquefied in a cryogeniccarbon dioxide purification stage, and the liquid is separated off fromwhich a gas stream having an elevated carbon dioxide content (carbondioxide gas stream) is obtained by reevaporation. In addition, a gasstream having a reduced carbon dioxide content (vent gas stream) isobtained from the non-liquefied raw gas. This vent gas stream isexpanded in at least one expansion turbine and the refrigerationgenerated in this process is recovered for cooling the raw gas stream.The carbon dioxide gas stream is compressed to a final pressure and fedto further utilization and/or storage. The invention also relates to anapparatus for carrying out the above-described method.

Carbon dioxide-containing gas streams are produced in all large-scalefurnace plants which are operated with fossil fuels such as coal, oil ornatural gas. These include, in particular, power plants, but alsoindustrial furnaces, steam kettles and similar large-scale thermalplants for power and/or heat generation. In addition, carbondioxide-containing gas streams are also formed in process plants of thechemical or petrochemical industry, such as cracking furnaces of olefinplants or steam reformers of synthesis gas plants. Owing to the harmfulclimatic effect of carbon dioxide gas, solutions are being sought inorder to decrease the emissions of carbon dioxide-containing exhaustgases into the atmosphere.

Very recently, novel power plant concepts are proposed in which thefossil fuel, e.g. coal, is burnt with an oxygen-rich combustion gas, inparticular with technically pure oxygen, or with oxygen-enriched air(oxygen combustion gas method). The oxygen fraction of this combustiongas is, e.g., 95 to 99.9% by volume. The resultant exhaust gas, which isalso termed flue gas, contains principally carbon dioxide (CO₂) at afraction of approximately 70 to 85% by volume. The purpose of thesenovel concepts is to inject the carbon dioxide formed in the combustionof the fossil fuels and present in concentrated form in the flue gasinto suitable repositories, in particular into certain rock strata orsalt water-bearing strata, and thereby to limit the emission of carbondioxide into the atmosphere. The harmful climatic effect of greenhousegases such as carbon dioxide is thereby reduced. Such power plants aretermed in the specialist field “oxyfuel” power plants.

In the concepts known to date, a dedusting, denitrification anddesulphurization of the flue gas proceed in sequential steps.Subsequently to this flue gas purification, the carbon dioxide-richexhaust gas thus treated is compressed and fed to a carbon dioxidepurification stage. There, typically, a gas substream having a reducedcarbon dioxide content and another gas substream having an elevatedcarbon dioxide content are generated by way of a cryogenic separationmethod. The gas substream having an elevated carbon dioxide content isthe desired carbon dioxide product stream, which is produced having acarbon dioxide content of, e.g., greater than 95% by volume and isprovided for further use, in particular for transport to repositories.The gas substream having a reduced carbon dioxide content is produced asa subsidiary stream (what is termed vent gas) at 15 to 30 bar,preferably 18 to 25 bar, and contains predominantly the components notintended for the injection, in particular inert gases such as nitrogen(N₂) and argon (Ar) and oxygen (O₂). However, in this gas substream,fractions of carbon dioxide are also still present at a concentration ofapproximately 25 to 35% by volume. This vent gas is currently blown offinto the atmosphere.

Usually, the raw gas stream is precompressed to a desired pressure inupstream plant parts and dried, e.g., in adsorber stations. This meansthat the vent gas also is first still present in the compressed state.At present this pressure level is reduced by expansion valves.

In EP 1952874 A1 and EP 1953486 A1 (Air Products) it has already beenproposed, after warming the vent gas and further heating by means ofwaste heat from the compression, to carry out a turbine expansion of thevent gas stream. However, utilization of the energy liberated in theturbine expansion, in particular of the refrigeration capacity producedin the expansion process, is not envisaged in this case.

In DE 102009039898 A1 (Linde), for improving the energy efficiency, itis proposed that the vent gas stream is expanded in at least oneexpansion turbine and both the resultant kinetic energy and also therefrigeration generated are utilized for energy recovery. For utilizingthe kinetic energy, the expansion turbine can be coupled to a compressor(booster) which compresses the raw gas stream and/or the carbon dioxidegas stream. For utilizing the refrigeration generated in the expansion,the at least partially expanded vent gas stream can be brought into heatexchange with process streams that are to be cooled, e.g. the raw gasstream and/or the carbon dioxide gas stream.

The carbon dioxide gas stream is usually compressed by way of a finalcompressor to the required final pressure of above 80 bar (preferably120 to 150 bar) for transport and subsequent sequestration.

Alternatively, for the carbon dioxide gas stream compression,liquefaction of the separated-off carbon dioxide-rich gas withsubsequent pressure elevation by way of pumps is also possible. Here,however, the use of refrigerant is necessary. When externalrefrigeration from a refrigeration plant is used, a liquid carbondioxide pure product is already present after the cryogenic separation,which liquid carbon dioxide pure product can be brought to the necessaryfinal pressure by way of a pump. However, the use of a refrigerationplant (external refrigeration) increases the necessary energyconsumption.

An object of the present invention is to design a method of the typementioned at the outset and also an apparatus for carrying out themethod in such a manner that the energy efficiency can be furtherimproved.

Upon further study of the specification and appended claims, otherobjects and advantages of the invention will become apparent.

These objects are achieved in terms of the method in that, from some ofthe liquid that is separated off in the cryogenic carbon dioxidepurification stage, a liquid stream having an elevated carbon dioxidecontent (carbon dioxide liquid stream) is obtained which is fed in aliquid phase to further utilization and/or storage.

Using the invention, an energy-sparing operation of the carbon dioxidepurification stage is made possible without using external refrigerationfrom an external refrigeration plant. The refrigeration necessary forcooling and partial condensation of the raw gas stream can be providedvia heat exchange with the evaporating liquid forming the carbon dioxidegas stream and also by heat exchange with the vent gas stream that iscooled by expansion in the expansion turbine. By branching off a carbondioxide liquid stream, the final compression of the carbon dioxide gasstream can be relieved, whereby, in total, an improvement of the energyefficiency is achieved. Furthermore, in this manner an additional liquidcarbon dioxide product can be provided without further energyconsumption.

It has proved in this case that operation of the carbon dioxidepurification stage without an external refrigeration plant is expedientwhen the carbon dioxide liquid stream amounts to 5 to 25%, preferably 10to 15%, of the total liquid that is separated off in the cryogeniccarbon dioxide purification stage.

According to a particularly advantageous embodiment of the invention,the carbon dioxide liquid stream is fed to the carbon dioxide gas streamafter compression thereof to the final pressure. In this case the carbondioxide liquid stream is expediently compressed to the final pressure byway of a liquid pump before it is fed to the carbon dioxide gas stream.

Another variant of the invention provides that the carbon dioxide liquidstream is temporarily stored in a liquid gas tank for further use, inparticular in the food industry. Customers' wants can thereby be met byan additional liquid carbon dioxide product without additional energyexpenditure and without the use of an external refrigeration plant.

A further possibility of use is in the use of the carbon dioxide liquidstream, after evaporation, as transport medium for the pneumatictransport (i.e., entrainment of solid particles, such as coal dust, by agas stream) or as a lock gas of feedstocks, in particular coal dust, inlarge-scale furnace plants. For example, during preparation ofcombustible material (coal/lignite) for coal fired power plants, contactwith oxygen has to be minimized. Carbon dioxide is possible gas to usefor displacing air, thereby acting as a lock gas.

For utilization of the kinetic energy, the expansion turbine can also becoupled to at least one compressor (booster), in such a manner that theexpansion turbine compresses the raw gas stream and/or the carbondioxide product stream during the at least partial expansion of the ventgas stream. For utilization of the refrigeration generated in theexpansion, the at least partially expanded vent gas stream is preferablybrought into heat exchange with process streams that are to be cooled,e.g. the raw gas stream and/or the carbon dioxide product stream. Byexpansion of the vent gas, process-internal refrigeration output can beprovided and external refrigeration can thereby be spared.

The invention further relates to an apparatus for treating a carbondioxide-containing gas stream (raw gas stream), in particular from alarge-scale furnace plant, having a carbon dioxide purificationappliance that is charged with the precompressed raw gas stream. Thecarbon dioxide purification appliance comprises an outlet line for a gasstream having an elevated carbon dioxide content (carbon dioxide gasstream) and an outlet line for a gas stream having a reduced carbondioxide content (vent gas stream). The outlet line for the carbondioxide gas stream is connected via a final compressor to a utilizationappliance and/or repository, whereas the outlet line for the vent gasstream is connected to at least one expansion turbine which comprises anoutlet line for the at least partially expanded vent gas stream. Theoutlet line for the at least partially expanded vent gas stream isconnected to a heat-exchange appliance which is chargeable with theprecompressed raw gas stream, the carbon dioxide gas stream and the ventgas stream.

The objects are achieved in terms of the apparatus in that the carbondioxide purification appliance additionally comprises an outlet line fora liquid stream having an elevated carbon dioxide content (carbondioxide liquid stream). This outlet line bypasses the heat-exchangeappliance and the final compressor and is connected directly to autilization appliance and/or storage appliance for liquid having anelevated carbon dioxide content.

Preferably, the outlet line for the carbon dioxide liquid streamcomprises a liquid pump and, downstream of the final compressor, thecarbon dioxide liquid stream is introduced into the outlet line for thecarbon dioxide gas stream.

The invention is suitable for all conceivable large-scale furnace plantsin which carbon dioxide-containing gas streams are produced. Theseinclude, e.g., fossil-fuel-fired power plants, industrial furnaces,steam kettles and similar large-scale thermal plants for power and/orheat generation. The invention can be used particularly advantageouslyin large-scale furnace plants which are supplied with technically pureoxygen or oxygen-enriched air as combustion gas and in which,accordingly, exhaust gas streams having high carbon dioxideconcentrations are produced. In particular, the invention is suitablefor what are termed low-CO₂ coal power plants which are operated withoxygen as combustion gas (“oxyfuel” power plants) and in which thecarbon dioxide that is present in the exhaust gas in high concentrationis separated off and injected below ground (“CO₂ capture technology”).

The invention is associated with a large number of advantages.

The final compressor for the carbon dioxide gas stream is relieved,which leads to energy savings and also to a reduction of capital costs.The demands on the operation of the final compressor are reduced so, forexample, a smaller compressor can be used thereby reducing capitalcosts. In addition, the energy balance at the heat-exchange appliance isoptimally utilized. In some circumstances, the intake temperature at thefinal compressor can be adjusted in such a manner that simpler materials(no high-alloy steels) can be used. Furthermore, a prepurified liquidcarbon dioxide product can be provided from the system which can beutilized, for example, for treatment to give a food-specific carbondioxide product (external utilization) or else also as liquid store in atank system.

The carbon dioxide liquid stream can also be used for other applications(e.g. treatment to give purified seal gas for the oxyfuel process, useas transport medium for the pneumatic transport of coal dust in theoxyfuel process, storage of liquid carbon dioxide for use as start-upgas or charge gas after evaporation).

When multistage compression is used to bring the pressure of the carbondioxide gas stream to the required final pressure, the carbon dioxideliquid stream, after a supercritical compression of the carbon dioxidegas stream (>72 bar), can be fed (in the supercritical state) upstreamof the suction side of the next-following compressor stage (or pump).The temperature falls and the density increases thereby. Owing to thehigher density, the energy requirement of the subsequent compressorstages/pumps for achieving the required final pressure falls.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further embodiments of the invention are described inmore detail hereinafter with reference to working examples shownschematically in the figures, in comparison with the previous prior art.Various other features and attendant advantages of the present inventionwill be more fully appreciated as the same becomes better understoodwhen considered in conjunction with the accompanying drawings, in whichlike reference characters designate the same or similar parts throughoutthe several views, and wherein:

FIG. 1 shows a block diagram of a carbon dioxide treatment plant withexpansion of the vent gas via an expansion turbine as per the prior artaccording to EP 1952874 A1;

FIG. 2 shows a block diagram of a carbon dioxide treatment plant withexpansion of the vent gas via expansion turbines with energy recovery asper the prior art according to DE 102009039898 A1;

FIG. 3 shows a block diagram of a carbon dioxide treatment plant withremoval of a separate carbon dioxide liquid stream with subsequentcompression by way of a liquid pump to the required final pressure forpipeline transport; and

FIG. 4 shows a block diagram of a carbon dioxide treatment plant withremoval of a separate carbon dioxide liquid stream with subsequenttemporary storage for external use.

FIG. 1 shows a conventional treatment of a carbon dioxide-containing rawgas stream from a coal power plant as per the prior art according to EP1952874 A1 (Air Products). The raw gas stream, after compression in theraw gas compressor 1, is fed via a heat-exchange unit 2 to a primaryseparator 3 for separating off carbon dioxide. The vent gas from theprimary separator 3 is introduced into the heat-exchange unit 2 and thenfed to a secondary separator 4. The carbon dioxide product stream iswithdrawn, respectively, from the bottoms of primary carbon dioxideseparator 3 and secondary separator 4, introduced into the centralheat-exchange unit 2 (and in the case of the vent gas from the primaryseparator 3 additionally via a CO₂ product compressor 7), and thensubjected to a final compression 8 in order finally to be fed via apipeline (carbon dioxide pipeline) 9, e.g. to an injection below ground.The vent gas of the secondary separator 4 is withdrawn from the top ofthe secondary separator 4, likewise introduced into the centralheat-exchange unit 2 and finally, downstream of a further warming in theheat exchanger 5, expanded via a turbine 6 in order to be delivered tothe atmosphere.

In contrast to the method shown in FIG. 1 for carbon dioxide treatment,the method according to DE 102009039898 A1 (Linde), shown in FIG. 2,offers the advantage of energy recovery in the expansion of the ventgas. In this method, as in the method shown in FIG. 1, two carbondioxide separators 3 and 4 and also a central heat-exchange unit 2 areprovided. In contrast to the method as per FIG. 1, however, simpleexpansion of the vent gas via a single turbine does not occur, butinstead a stepwise expansion via two expansion turbines 5 and 6 isperformed. By way of the stepwise expansion of the vent gas stream, theformation of solid carbon dioxide in the vent gas can be prevented.Downstream of the expansion in the first expansion turbine 5, the ventgas stream is warmed in the central heat-exchange unit 2 and thenexpanded further to close to atmospheric pressure in the secondexpansion turbine 6 and again warmed in the central heat-exchange unit2. The available pressure level of the vent gas can thereby becompletely exploited. The vent gas that is cold after the expansion iswarmed in the central heat-exchange unit 2 against the process streamsthat are to be cooled. The vent gas thereby provides some of therefrigeration capacity necessary in the process.

FIG. 3 shows a carbon dioxide treatment according to the invention. Theprocess procedure differs from that shown in FIG. 2 in that some of theliquid carbon dioxide separated off in the primary separator 3 isbranched off and fed via a carbon dioxide liquid pump 9 downstream ofthe final compressor 8 to the CO₂ pipeline 10. In this procedure thecarbon dioxide liquid stream is brought to the required final pressurefor the pipeline transport by way of the carbon dioxide liquid pump 9.Since this carbon dioxide liquid stream by passes the final compressor8, the operational demands on the final compression 8 are reducedthereby increasing energy efficiency and reducing capital costs. Theenergy balance at the heat-exchange unit 12 can be used optimally. Theintake temperature at the final compressor 8 can be adjusted in such amanner that simpler materials (e.g. no high-alloy steels) can be used.

In the variant of the invention shown in FIG. 4, the liquid carbondioxide that is separated off from the primary separator 3 is firsttemporarily stored in a liquid carbon dioxide tank 11. The liquid carbondioxide can be fed, as required, from tank 11 via the liquid pump 9 tothe CO₂ pipeline 10 and/or loaded into a transport vehicle 12 forfurther external utilization (e.g. in the food industry) and/orevaporated in a CO₂ evaporator 13 and provided for internal utilization(e.g. as start-up or feed gas).

The entire disclosure[s] of all applications, patents and publications,cited herein and of corresponding German Application No. 10 2011 014678.4, filed Mar. 22, 2011, are incorporated by reference herein.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

1. A method for treating a carbon dioxide-containing gas streamcomprising: partially liquefying precompressed raw carbondioxide-containing gas stream in a cryogenic carbon dioxide purificationstage, removing from said cryogenic carbon dioxide purification stage aliquid stream having an elevated carbon dioxide content and a gas streamhaving a reduced carbon dioxide content, reevaporating a first portionof the liquid stream having an elevated carbon dioxide content to obtaina gas stream having an elevated carbon dioxide content, expanding thegas stream having a reduced carbon dioxide content in at least oneexpansion turbine and recovering the resultant refrigeration generatedin this expansion for cooling the precompressed raw carbondioxide-containing gas stream, compressing the gas stream having anelevated carbon dioxide content to a final pressure, and feeding theresultant compressed gas stream having an elevated carbon dioxidecontent to further utilization and/or storage, wherein a second portionof the liquid stream having an elevated carbon dioxide content removedfrom said cryogenic carbon dioxide purification stage is fed in a liquidphase to further utilization and/or storage.
 2. The method according toclaim 1, wherein said precompressed raw carbon dioxide-containing gasstream is a raw gas stream form a large-scale furnace plant.
 3. Themethod according to claim 1, wherein the second portion of the liquidstream having an elevated carbon dioxide content is 5 to 25% of thetotal liquid removed from the cryogenic carbon dioxide purificationstage.
 4. The method according to claim 3, wherein the second portion ofthe liquid stream having an elevated carbon dioxide content is 10 to 15%of the total liquid removed from the cryogenic carbon dioxidepurification stage.
 5. The method according to claim 1, wherein thesecond portion of the liquid stream having an elevated carbon dioxidecontent is introduced into the gas stream having an elevated carbondioxide content after compression of the second portion of the liquidstream having an elevated carbon dioxide content to the final pressure.6. The method according to claim 5, wherein the second portion of theliquid stream is compressed to the final pressure by way of a liquidpump before it is introduced into the gas stream having an elevatedcarbon dioxide content.
 7. The method according to claim 1, wherein saidsecond portion of the liquid stream having an elevated carbon dioxidecontent is temporarily stored in a liquid gas tank for further use. 8.The method according to claim 3, said second portion of the liquidstream having an elevated carbon dioxide content is used as transportmedium for the pneumatic transport of feedstocks.
 9. An apparatus fortreating a carbon dioxide-containing gas stream, said apparatuscomprising: a carbon dioxide purification appliance comprising an inletfor introducing a precompressed raw carbon dioxide-containing gasstream, an outlet line for removal of a gas stream having an elevatedcarbon dioxide content and another outlet line for removal of a gasstream having a reduced carbon dioxide content, said outlet line forremoval of a gas stream having an elevated carbon dioxide content beingconnected via a final compressor to a utilization appliance and/orrepository, said another outlet line for removal of a gas stream havinga reduced carbon dioxide content being connected to at least oneexpansion turbine comprising an outlet line for removal of at leastpartially expanded gas stream having a reduced carbon dioxide contentwhich is connected to an inlet of a heat-exchange appliance, and saidheat exchange appliance further having inlets for introduction of saidprecompressed raw gas stream and said gas stream having an elevatedcarbon dioxide content, wherein said carbon dioxide purificationappliance additionally comprises an outlet line for a liquid streamhaving an elevated carbon dioxide content, which bypasses saidheat-exchange appliance and said final compressor, and is connecteddirectly to a utilization appliance and/or storage appliance for liquidhaving an elevated carbon dioxide content.
 10. The apparatus accordingto claim 9, wherein said outlet line for a liquid stream having anelevated carbon dioxide content comprises a liquid pump and, downstreamof said final compressor, outlet line for a liquid stream having anelevated carbon dioxide content communicates with said the outlet linefor removal of a gas stream having an elevated carbon dioxide contentbeing.